Device for absorption of sound waves

ABSTRACT

A device having a high sound absorption ability and intended for use as floor, wall and/or ceiling facing in so-called echo-free rooms (10), i.e. in acoustic measuring rooms designed according to international standard, ISO-3745, in picture and sound recording studios, in sound laboratories etc., in which a sound propagation as in a free field is desired. The new device is based on sound absorbents in the form of plates (16, 20) forming a substrate support (16) and units (20) projecting obliquely from this which are so placed that they have a wave-like cross-section. The size and reciprocal angular conditions of the sound absorbents (20) projecting from the substrate support (16) are chosen so that at least double reflection is always obtained before a sound wave deriving from a measuring object (12) is directed from the device. The sound absorption ability is further increased by utilizing a low-frequency cavity resonance between the projecting sound absorbents (20) and the substrate support (16).

This invention relates to a device for absorbing sound waves andintended for use as an internal facing, primarily in so-called echo-freerooms, in which recording and measurement of sound are carried out bymeans of sensitive measuring instruments.

Devices of said type are suited for use in all rooms where a high degreeof reflection freedom is desired, e.g. in measuring rooms made inaccordance with international standard, ISO 3745, in sound and picturerecording studios, in sound laboratories etc. In all these rooms a soundfield is desired which corresponds to free sound wave propagation, thestrenght of the sound reflex in the limiting surfaces being very small.According to the ISO-standard a sound absorption ability of 99.9% isrequired within the current frequency range, but in certain cases asound absorption ability of 99.0% in a medium high sound frequency rangefrom e.g. 200 Hz is sufficient.

A device of the intended type should be well suited as facing both onwalls, in ceilings and on floors.

Previously known constructions for absorption of sound waves inecho-free rooms comprise primarily facings in the form of wedges ofblocks and wads or cubes of sound absorbing material suspended inthreads in immediate connection with walls, ceilings and floors. Thepurpose of the used constructions is to achieve impedance adaption ofthe propagation medium, i.e. air, in the intended room in order toobliterate in this way the limit of the propagation of the sound waves.

The known constructions have many shortcomings. Thus, they requireseparate, specially designed building elements the manufacture of whichis expensive. These building elements are often difficult to assemble,especially in connection with floors and ceilings and therefore involvea time-consuming work. Moreover, it is difficult and in certain casesimpossible to clean them. It is also difficult to replace damagedbuilding elements of this type. As the building elements compriseunprotected mineral fiber material there is also a great risk of fiberscoming loose, the environment of the room being deteriorated.

It is now the object of the invention to provide a sound absorptiondevice eliminating all the disadvantages in connection with knownconstructions, particular stress being laid on utilizing simple buildingelements easy to mount and giving a very high sound absorption. This isachieved in that the absorption device of the invention comprises asubstrate support which e.g. consists of mineral wool sheets or plate towhich plate-shaped sound absorbents, preferably of mineral wool areinclined relative to each other in such a way that they will have awave-shaped cross-section. The angle between two sound absorbents placedon the substrate support with their edges close to each other should beeasily adjustable and is chosen in such a way that the sound wavesderiving from a sound source are always reflected at least twice againstthe sound absorbents before they leave the device. In order to obtainfurther improvement of the sound absorption ability at the lower portionof the intended frequency range the substrate support together with twoadjacent sound absorbents placed on the substrate support should,moreover, define a cavity with a gap-shaped opening disposed between thesound absorbents and/or between sound absorbent and substrate support,which opening is so adapted that cavity resonance is obtained at apredetermined lower frequency range.

A sound absorption device built in the abovementioned manner can utilizeusual rectangular plane mineral wool plates of a standard format assound absorbents. The width of the plates is chosen in dependence ofdesired lower limit frequency of the operating range of the soundabsorbents.

The invention will now be described more in detail below in the form ofa preferred illustrative example with reference to the accompanyingdrawing.

FIG. 1 is a top plan view of a section of an echo-free room comprisingthe sound absorption device of the invention.

FIGS. 2A and 2B disclose on an enlarged scale a part of the soundabsorption device illustrated in FIG. 1.

FIGS. 3A and 3B show diagrams of measuring lines of a measuring objectcentrally located in a room at test measurements according toInternational Standard ISO 3745.

FIG. 4 shows the embodiment of a result diagram used to obtain theresult evaluated by the aid of a computer of measurements made accordingto the measuring diagrams in tables 1-4.

The echo-free room 10 illustrated in FIG. 1 is especially intended formeasurements of noise of car engines, the position of the measuringobject 12 for obtaining the best measuring result being limited to thecentral part of the room 10. On the drawing sound absorbing devices aredisclosed only in connection with the walls 14 of the room 10 butsimilar devices should of course also be arranged in the ceiling of theroom 10 and optionally also in connection with its bottom portion. Inthe latter case the real supporting floor surface consists of alattice-work placed above the sound absorbents projecting from thebottom portion.

As is apparent from the drawing, especially FIG. 1, the room 10 isinternally covered with a substrate support 16 of sound absorbentmaterial. This consists preferably of mineral wool plates fixed to therelative wall 14 in a way known per se, e.g. by means of round metalwires (not shown) passing through. In the four corners of the room 10there are special air drums 18 through which fresh air is fed into theroom. There are also similar arrangements not shown close to the ceilingto divert off-air. All the air drums are covered by substrate supports16 of sound-absorbing material of the same type as that used for thewalls 14.

Plate-shaped sound absorbents 20 project from the walls 14. These areplaced on edge two by two close to each other on the substrate support16 so that a predetermined angle is formed between them. The soundabsorbents 20 extend from floor to ceiling, which means in reality thatseveral sound absorbents 20 are placed above each other edge to edge. Inthe arrangement shown the sound absorbents 20 are vertically orientedbut this is no demand. The sound absorbents 20 in each pair have at theattachment to the substrate support 16 their adjacent edges placed closeto each other. The oppositely located edges of the sound absorbents ofadjacent sound absorbent pairs have a reciprocal interspace as isespecially apparent from FIG. 2A.

The angles between the sound absorbents 20 in each pair of soundabsorbents are so chosen that sound waves deriving from the measuringobject 12 are always reflected at least twice against the soundabsorbents 20 before they are again directed to the room 10. In this waya sound absorption of at least 99% is obtained as the sound absorptionability of the sound absorbents 20 chosen in the present case, i.e. themineral wool plates, which are of standard type, amounts of between 90and 95%. For the mutual fixation of the sound absorbents 20 and fortheir mounting onto the substrate support 16 perforated plate stripes ofductile stretch metal bands 22, 24 (see FIG. 2A) known per see areutilized which run along and overlap the edges of the sound absorbents20 facing each other and do not block the way of the sound wavesappreciably. Besides said stretch metal bands 22, 24 round zinc threads26, 28 are also used for the reciprocal fixation of the sound absorbents20, which threads run through the bands 22, 24 and the sound absorbents20, as is especially evident from FIG. 2. These fixing threads 26, 28are of the same type as the threads anchoring the sound absorbents 20 tothe substrate support 16.

At the edges of the sound absorbents 20 facing away from the substratesupport 16 an adjustable desired width of the gap 30 is ensured by acorresponding bending of the relative stretch metal band 22. Twoadjacent sound absorbents 20 which are placed against the substratesupport 16 with their edges spaced from each other define together withthe substrate support 16 a cavity 32, the cross-section of which istriangular. The width of the slot-shaped gap 30 is so chosen relative tothe volume of the cavity 32 that a low frequency cavity resonance isobtained immediately below the selected lower limit frequency, e.g. 200Hz.

Although not especially apparent from the drawing each sound absorbent20 is enclosed in a stretch bag or sock which can be made of nylonfabric. By this arrangement the mineral wool fibers are prevented fromcoming loose from the sound absorbents 20 when these are exposed toblows and stresses of different kind. Thus, the stretch sock holds thefibers together and at the same time it is an outer casing which is easyto wipe off and clean. In addition this outer casing can be dyed asdesired, so that the echo-free room 10 can be made aestheticallyattractive.

On the drawing some doors are also shown, one 34, through whichmeasuring objects are transported in and out, and one 36 leading to anouter observation room. The latter is also provided with a window 38through which the measuring object 12 can be inspected. Sound absorbingdevices are placed as close to doors and windows as possible and,moreover, directly against the inside of the relative doors.

Modifications of the construction described above can of course be madewithin the scope of the invention. Instead of arranging a gap 30 betweenthe sound absorbent 20, as shown in FIG. 2A, a corresponding gap 30' canbe arranged between the sound absorbent 20' and the substrate support16', as shown in FIG. 2B.

It is also possible to arrange blowing of uniformly distributed air intothe room from said cavities 32, 32', via the corresponding gaps 30 and30', respectively.

I claim:
 1. A device intended for absorption of sound waves and for useas internal facing, primarily in so-called echo-free rooms, said devicecomprising a substrate support which consists of mineral wool plates, towhich plate-shaped sound absorbents, preferably of mineral wool, areinclined relative to each other in such a way that they have a wave-likecross-section, wherein the angle between two sound absorbents placedwith their edges close to each other on the substrate support is sochosen that the sound waves deriving from a measuring object are alwaysreflected at least twice before they leave the device, and wherein thesubstrate support as well as two adjacent sound absorbents, which areplaced on the substrate support with their edges in spaced relationshipfrom each other, define a cavity with a gap between the sound absorbentsand/or between sound absorbents and substrate support, which gap is soadapted that cavity resonance is obtained at a predetermined lower rangeof frequencies.
 2. The device of claim 1, characterized in that thewidth of the sound absorbent is chosen in dependence of desired lowerlimit frequency of the operating range of the sound absorbent.
 3. Thedevice of claim 1, characterized in that each sound absorbent isenclosed in a stretch bag or sock of nylon fabric or the like.
 4. Asound-absorbing internal facing for an echo-free room comprising asheet-like substrate of sound-absorbing material, the substrate having asurface adapted to face inwardly when the substrate is attached to awall ceiling or floor of a room, and plate-shaped elements ofsound-absorbing material arranged on said surface of the substrate andinclined relative to each other and to said surface so as to have awave-like cross-section, the angle between adjacent plates which divergefrom each other in a direction away from the substrate being such thatsound waves from a measuring object spaced from said surface arereflected at least twice before they leave said plates, and adjacentplates which converge toward each other in a direction away from thesubstrate forming a cavity bounded by said converging plates and thesubstrate, the edges of said converging plates remote from saidsubstrate being spaced apart so as to form a gap between them such thatcavity resonance is obtained at a predetermined lower range offrequencies.
 5. A sound-absorbing internal facing for an echo-free roomcomprising a sheet-like substrate of sound-absorbing material, thesubstrate having a surface adapted to face inwardly when the substrateis attached to a wall ceiling or floor of a room, and plate-shapedelements of sound-absorbing material arranged on said surface of thesubstrate and inclined relative to each other and to said surface so asto have a wave-like cross-section, the angle between adjacent plateswhich diverge from each other in a direction away from the substratebeing such that sound waves from a measuring object spaced from saidsurface are reflected at least twice before they leave said plates, andadjacent plates which converge toward each other in a direction awayfrom the substrate forming a cavity bounded by said converging platesand the substrate, the edges of at least one of said converging platesadjacent said substrate being spaced from said substrate so as to form agap between said edge and said substrate such that cavity resonance isobtained at a predetermined lower range of frequencies.